When an ultrasonic pulse is radiated toward a body to be examined, an echo returns from a reflection material inside of the body. Therefore, a tomographic image of the body under examination can be obtained if the strength of the echo is B mode displayed. Further, when the reflection material is motional, the frequency of the echo shifts from the transmission frequency due to a Doppler effect. By using this phenomenon, it is capable of knowing, for example, the direction and speed of a blood flowing in a heart or a blood vessel.
While the transmitting ultrasonic beam is required to have a sharpness to form a B mode image of excellent resolution, it is not necessarily required to have a sharpness in the Doppler mode, since in the latter mode utilized in an echo of a sample volume of a particular magnitude that is reflected from the body under examination. The magnitude of the sample volume is determined appropriately depending upon an object to be observed. An adjustment of the sample volume is carried out by adjusting the width of a range gate for use in receiving the echo. In the adjustment of the sample volume, the dimension of the sample volume in the depth direction increases as the width of the range gate is widened; however, the dimension in the direction perpendicular thereto, i.e. azimuth direction of the ultrasonic beam, cannot be changed, since it has been fixed in accordance with a profile of the ultrasonic beam. That is, there has heretofore been no freedom in setting the sample volume in the azimuth direction.
Recently, in the field of ultrasonic medical equipment, limitation regarding an amount of ultrasonic radiation has been strictly reduced. The pulse Doppler is a typical case &hat is liable to be contrary to the regulation concerning the amount of radiation, since due to the necessity for continuously observing a specific position, the ultrasonic beam of a particular sound-ray is transmitted and received at a maximum pulse repetitive frequency (PRF) allowable for the depth of the portion in question. In a conventional ultrasonic diagnostic apparatus provided with both the B mode and the Doppler mode diagnostic functions, a common beam forming data is used in forming the transmission beams for both the B mode and the Doppler mode, so that the ultrasonic beam radiated in the Doppler mode is as sharp as that in the B mode, thereby making it difficult to act upon the regulation.